Upon recent abnormal weather or a disaster such as a typhoon or a flood, conventional land-based pumps sometimes cannot be operated when they are required to work. Introduction of submersible pumps, which can be operated even when they are submerged, is a conceivable countermeasure, and replacement of a land-based pump with a submersible pump at the time of pump replacement is increasing these days.
The introduction of a submersible pump necessitates the use of a submersible motor having an unprecedentedly large capacity. In particular, some overseas rivers and drainage pump stations are of a large scale and, in most cases, a high-power (1 MW-class) submersible motor is used for them.
Submersible motors are becoming higher-voltage ones with the progress toward higher-power submersible pumps. This is because of the following disadvantages of low-voltage submersible motors: In order to pass a high current through a submersible motor, it is necessary to use a large-sized cable to connect the motor body of the submersible motor with a power source. The use of a large-sized cable involves an increased cable cost and an increased cable weight and, in addition, may necessitate the use of a special cable. Such a cable is difficult to handle. Further, a large voltage drop will occur in such a submersible motor. This requires increase in the size of a cable and increase in the starting capacity of the submersible motor. Thus, the size of a generator and the size of a transformer must be increased and, accordingly, the size of a facility and the size of a breaker must be increased. Furthermore, the use of such a submersible motor necessitates a large-sized connecting portion of the motor body and a cable, leading to a difficult connecting operation.
High-voltage submersible motors, on the other hand, have the following advantages: A high-voltage submersible motor can reduce an electric current, and therefore can reduce the size of a cable. Further, a high-voltage submersible motor can reduce a voltage drop, and therefore can reduce the starting capacity. This enables a reduction in the size of a facility and the size of a breaker. In addition, the use of such a submersible motor can reduce the size of a connecting portion of the motor body and a cable, thereby reducing the cost of the submersible motor and facilitating a connecting operation.
There is known a submersible motor of the type that electrically connects a cable to the motor body via a waterproof connector. FIG. 13 is a diagram showing a connector-type submersible motor. As shown in FIG. 13, a cable 137 is connected to a waterproof connector 146. The cable 137 is composed of a power cable 137a and an earth cable 137b. The power cable 137a and the earth cable 137b are connected to a cable contact 150 and an earth contact 151, respectively.
The waterproof connector 146 includes an insulator 149 to which the cable contact 150 and the earth contact 151 are liquid-tightly connected, and a conductive cover member 148 to which the insulator 149 is liquid-tightly connected. A lead wire 145 is connected to a motor body 136, while the lead wire 145 is connected to a motor contact 142. The motor contact 142 is liquid-tightly connected to an insulator 141 which in turn is liquid-tightly connected to a motor casing 140. When the cable contact 150 is connected to the motor contact 142, power is supplied from a power source to the motor body 136 through the cable 137 and the lead wire 145.